is useful to set the net mass of one or more components in the model
to a known value;
is useful to scale the net mass of one or more components in the model
(without the need to know the mass of the component);
is useful to account for any errors in mass due to modeling
approximations;
is useful to account for mass from nonstructural features otherwise
omitted from the model, such as paint;
can be applied over all element types that have mass;
adjusts the mass of the individual elements in an element set in
proportion to their pre-adjusted mass including any nonstructural mass, so as
to meet the specified target value for the set;
can be used to redistribute mass among elements in the set to raise
the minimum stable time increment to a target value;
can be used to redistribute mass to maximize the stable time increment
of the elements in the element set;
can be specified only once in an
Abaqus/Explicit
analysis during the model definition; and
can be applied in a hierarchical fashion to adjust the mass for
individual parts first and then for an assembly of these parts.
To adjust or scale the total mass of one or more components in the model,
you first identify the corresponding element sets. If you specify multiple
elements sets, the mass is adjusted or scaled in the order in which the element
sets are specified. For element sets that share elements, you must determine
the order in which to specify the element sets to obtain the desired results.
Adjusting the Total Mass of an Element Set to a Known Value
The mass of a component in a numerical model may differ from its actual
value for a number of reasons including modeling approximations and omission of
minor features from the model. You can specify mass adjustment in the numerical
model for such components by identifying the element sets defining these
components and their respective total mass values. For a given element set, the
mass is adjusted at the start of the analysis such that the adjustment in each
element in that set is in proportion to the pre-adjusted mass of that element,
thus preserving the center of mass and the principal directions of the rotary
inertia. The pre-adjusted mass of an element includes the mass due to any
associated material density; any mass directly specified on the section
definition as in the case of beam, pipe, shell, membrane, rigid, and surface
elements; and any nonstructural mass applied directly to that element.
Knee bolster impact with general contact
is an example of setting the total mass of an element set using mass
adjustment.
When mass is adjusted for an element with active rotational degrees of
freedom, the rotary inertia contribution from that element is also modified
proportionally to correspond with the scaling in the element mass from mass
adjustment, thus preserving the principal directions of the rotary inertia. The
adjusted mass value is considered when calculating the stable time increment of
an element. Loads such as mass proportional damping (see
Material Damping)
and gravity take the adjusted mass into account.
Mass adjustment can be applied in a hierarchical fashion to adjust the mass
for individual parts first and then for an assembly of these parts. In this
scenario, the mass adjustment defined over the assembly may further modify the
adjusted mass of the individual parts. You must associate all of the
mass-adjusted element sets in the desired order with a single mass adjustment
definition.
Abaqus/Explicit
automatically calculates the mass, center of mass, and rotary inertia of each
element set and prints the results to the data (.dat) file
if model definition data are requested (see
Controlling the Amount of analysis input file processor Information Written to the Data File).
The contributions from mass adjustment are also listed in these tables. Element
output variable MASSADJUST can be requested as output to the output database
(.odb) file, and it will indicate how the mass of the set
is adjusted or redistributed to each element included in the set (see
Abaqus/Explicit Output Variable Identifiers).
This output variable is available as field output (contour plots) in the first
output frame of the first analysis step.
Mass adjustment contributions applied to an element set are always included
when transferring model data between
Abaqus/Explicit
analyses (see
Transferring Results from One Abaqus/Explicit Analysis to Another).
There is no need to redefine these contributions in the import analysis unless
different mass adjustment is required for the element set.
Redistribution of Mass to Raise the Minimum Stable Time Increment to a Target Value
You can increase the minimum stable time increment in the initial
configuration for an element set to a specified target value by redistributing
mass among the elements in that set. The redistribution of mass to affect the
stable time increment and adjustment of mass to achieve a target total mass can
be requested independently of each other. If both options are requested for a
given element set, the mass is first adjusted to meet the target total mass for
the set and then redistributed among the elements to achieve the target time
increment.
You can set a default target time increment that is applicable for all of
the mass-adjusted element sets as well as specific targets for any of the
individual element sets. Within each set, the mass is transferred to the
elements with time increments below the target value from the remaining
elements.
Abaqus/Explicit
prints the amount of mass available for redistribution along with the
percentage of this amount that is redistributed to the data
(.dat) file if model definition data are requested (see
Controlling the Amount of analysis input file processor Information Written to the Data File).
If a sufficient amount of mass is not available to meet the specified target
time increment, the analysis terminates with an error message.
Impact of a water-filled bottle
is an example of maintaining the target stable time increment of an element set
using mass adjustment.
When compared to the fixed mass scaling functionality, the redistribution
feature above does not alter the total mass of the set. However, both features
affect the center of mass and the principal directions of rotary inertia. The
redistribution feature is performed only in the initial configuration at the
start of the analysis; whereas the fixed mass scaling is performed in the
configuration at the start of the step requesting that mass scaling. When you
specify mass adjustment and mass scaling, the mass scaling adds mass as
necessary on top of the adjusted mass.
Redistribution of Mass Using a Scale Factor
An alternative method to adjust the mass of an element set is to specify a
scale factor. When the mass adjustment is defined using a scale factor, you can
maximize the minimum stable time increment in the initial configuration for an
element set by redistributing either the total mass or only the added mass
among the elements in that set. The redistribution of the total mass to
maximize the time increment is done iteratively by taking mass from each
element with a stable time increment greater than the average and adding it to
the elements with lower time increments—this iterative process stops when all
elements have the same time increment within a tolerance.
The redistribution of only the added mass to maximize the time increment is
done in a similar fashion, but only the added mass is transferred between
elements. The elements may not have the same time increment after the
redistribution is complete. In this case the scale factor should be greater
than one. Both choices of mass redistribution described here affect the center
of mass and the principal directions of rotary inertia, but redistributing only
the added mass with a relatively small scale factor may have a lesser impact.
Alternatively, you can scale the mass to redistribute it uniformly (in
proportion with the pre-adjusted mass) without affecting the center of mass and
the principal directions of rotary inertia.